1. Field of the Invention
The present invention relates to a charging device, a process cartridge and an image forming apparatus, and more particularly to a charging device which charges an image bearing member by electron discharging, a corresponding process cartridge and a corresponding image forming apparatus.
2. Discussion of the Background
An image forming apparatus based on electrophotographic process is known to function as a printer, a facsimile machine, a photocopier, a plotter, and a multi-functional machine having functions of a printer, a facsimile machine and a photocopier. In this electrophotographic process, corona charging is widely adopted to uniformly charge an image bearing member to form a latent electrostatic image thereon.
This corona charging system is as follows: an electroconductive case electrode is provided around a wire electrode formed of platinum or tungsten having a diameter of from about 50 to about 200 μm or an electrode having a needle form made of a stainless material; DC or AC high voltage is applied between the electrode and the case to ionize air molecules around the electrode; and the image bearing member is charged by the ionized molecules. In this system, uniform charging can be performed from a distance.
However, in the corona charging system, air is ionized so that corona products such as ozone and nitrogen oxides are produced. The amount of the products for both of ozone and nitrogen oxides, is known to reach as high as 4 to 10 ppm after 60 minute charging.
It is known that when the density of ozone accumulated in an image forming apparatus is high, the surface of an image bearing member is oxidized, resulting in deterioration of the photosensitivity of the image bearing member and the charging power. Therefore, degraded images are obtained (refer to “Development of Corona Charger for the Reduction of the Bad Influence of Ozon on a Photoconductor” (The Imaging Society of Japan, ISJ), authored by Hisashi MYOCHIN, et. al., Vol. 31, 1, published in 1992). In addition, deterioration of devices other than an image bearing member is accelerated, resulting in problems such as short life of the devices.
Further, such nitrogen oxides cause the following problems. That is, the nitrogen oxides produce nitric acid in reaction with moisture in the air and metal nitrate in the reaction with a metal. These products have a high electric resistance in a low moisture environment but a low electric resistance in a high moisture environment when reacting with moisture in the air. Therefore, when a thin layer of nitric acid or metal nitrate is formed on the surface of an image bearing member, abnormal images such as a flowing image are obtained. This is because nitric acid or a salt thereof absorbs moisture so that the resistance of the surface of the image bearing member is low, resulting in destruction of a latent electrostatic image on the surface of the image bearing member.
Further, nitrogen oxides still accumulate in the air in the image forming apparatus without dissolving after discharging. The compound produced from the nitrogen oxides attaches to the surface of the image bearing member even when charging is not performed, that is, during idle periods of the process. Furthermore, the compound infiltrates from the surface to the inside of the image bearing member over time. This causes deterioration of the image bearing member.
In this case, the attachment on the surface of the image bearing member is removed by slightly scraping the image bearing member at a time during cleaning. However, this method involves new problems such as cost increase and deterioration over time.
In the corona charging, the applied voltage is significantly high, that is, from 4 to 10 kV, because the charging is performed from a distance. In addition, the charged voltage depends on the charging time. Therefore, when the linear velocity of an image bearing member is high, it is necessary to broaden the width of a case electrode along the rotation direction of the image bearing member. Therefore, the size reduction of an image forming apparatus having a high printing speed is difficult.
As another charging device, a charging system using a roller disposed in the vicinity of an image bearing member is now widely used. In the vicinity type roller charging, an image bearing member is charged such that an AC or DC bias is applied to between an image bearing member and a charging device (charging roller) disposed in the vicinity of the image bearing member to cause discharging in the space therebetween. In this charging system, the charging phenomenon based on Paschen's law is utilized. That is, a desired voltage is obtained by forming a voltage difference by a discharging starting voltage against the desired voltage.
In the AC bias system, the direction of the electric field formed between an image bearing member and a charging device disposed in the vicinity thereof alternates with time. Therefore, discharging and reversed discharging are repeatedly performed. Charging is evened out by discharging and reversed discharging in the AC bias system, resulting in uniform charging. However, there is a disadvantage that hazard to an image bearing member by discharging is extremely large.
The electron is provided to an image bearing member by a charging device involving Paschen discharging. As a result, hazard is inevitable. For example, products produced by discharging are attached to the surface of an image bearing member or the surface of an image bearing member is oxidized by an active air produced by discharging.
To deal with this drawback, as mentioned above, the surface of an image bearing member is minutely scraped at a time to reduce the deterioration of image quality over time. On the other hand, scraping the surface of an image bearing member is equal to attrition. It is naturally preferred to avoid such scraping from a long-term point of view. However, this scraping has a trade-off relationship with protection against image deterioration caused by the hazard to an image bearing member mentioned above. Therefore, it is difficult to find a fundamental solution to this drawback.
Further, there is another charging system, which is a contact type charging device. In this system, a charging member is brought into contact with an image bearing member to charge the image bearing member. For example, a charging member having a roller form charges an image bearing member by being driven by the image bearing member while in contact therewith. When compared with the corona charging system mentioned above, the contact type charging system has advantages such that the amount of ozone produced after 60 minute charging using DC is as small as 0.01 ppm, the applied voltage is low so that the cost of power is small and it is easy to design electric insulation.
As the contact type charging system, as described in unexamined published Japanese Patent Applications Nos. (hereinafter referred to as JOP) S57-178257, S56-104351, S58-40566, S58-139156, and S58-150975, there are methods in which discharging according to the interpretation of Paschen's law is performed at the contact portion or a narrow space formed in the vicinity thereof to charge an image bearing member. It is possible to accelerate uniform charging by applying a DC voltage not less than the charge starting voltage to an electroconductive portion or applying an oscillation voltage in which an AC voltage is overlapped with a DC voltage corresponding to the desired charging voltage, which is detailed in JOP 63-149669.
As mentioned above, when an AC voltage is applied, there is an advantage that, since the direction of electric field formed between an image bearing member and a charging device disposed in the vicinity thereof alternates with time, discharging and reversed discharging are repeatedly performed so that charging is evened out by discharging and reversed discharging. However, the amount of electric current increases. Therefore, the amount of ozone and nitrogen oxide produced increases as the electric current increases. Depending on the condition of AC application, the amount of ozone produced reaches almost 3 ppm after 60 minute charging, which is close to that in the corona charging system.
On the other hand, as described in JOP H08-106200, there is another method in which the electroconductive member mentioned above to which a voltage is applied is brought into contact with an image bearing member to infuse electrons on the surface of the image bearing member. As the electroconductive member in this method, an electroconductive member having a roller form (charging roller) is typically used in terms of easiness of controlling attachment/detachment and making a form.
However, since the charging member is formed of rubber, when, for example, a photocopier is idle for a long time, the roller, which is in contact with an image bearing member, may be transformed. In addition, since rubber is a material which easily absorbs moisture, the electric resistance thereof significantly varies according to changes in environment. Further, rubber requires several kinds of plasticizers and active agents to become elastic and prevent deterioration. Further, a dispersion helping agent is used to disperse electroconductive pigments in a considerable number of cases. That is, since the surface of an image bearing member is an amorphous resin such as polycarbonate and acryl resin, such an image bearing member is extremely weak to the plasticizers, active agents and dispersion helping agents mentioned above.
In addition, there are other problems as follows involving the contact-type charging system: when a foreign substance is nipped between a charging member and an image bearing member, the charging member is contaminated, which leads to occurrence of poor charging; and since the roller is in direct contact with an image bearing member, the image bearing member is contaminated overtime, which causes image degradation such as streak in the horizontal direction.
As a result, as a superseding technology, the system using an electron discharging material is getting attention. For example, JOP 2003-145826 describes a technology in which the electron discharging element of MIS type and MIM type is used. The electron discharging element has a structure in which an electron discharging layer formed of an insulation layer and a semiconductor material layer or metal material layer is sandwiched between a substrate electrode and a thin layer electrode.
JOP 2001-250467 describes a technology using an electron discharging element having a carbon nanotube. The peripheral part of the carbon nanotube is coated with metal or alloyed metal, or at least one compound selected from a nitride, a carbide, a silicide or a boride containing a metal.
JOP 2002-279885 describes an electron emission apparatus, a charging device and an image forming apparatus regarding an electron emission apparatus using carbon nanotube, which are capable of stable emission of electron in the atmospheric pressure and with low voltage.
JOP 2003-140444 describes a technology using an electron discharging element in which a semiconductor layer is formed between the top electrode and the bottom electrode. An organic compound absorption layer is formed so that an organic compound is absorbed in the semiconductor surface of the semiconductor layer.
Further, JOPs 2002-311684 and 2004-327084 describe technologies using an electron discharging element.
Among the electron discharging elements mentioned above, the carbon nano materials have been intensively studied. Among these, carbon nanotube has widely studied and it is suggested that carbon nanotube has a high electron discharging power. For example, JOP 2001-250467 describes that durability of carbon nanotubes can be improved by regulating the component for use in the peripheral part thereof and carbon nanotubes can be used as a contact type or non-contact type charging device.
However, there is a problem with carbon nano materials stemming from the fact that the carbon nano materials are organic compounds. That is, in the electrophotography system, electrons are discharged into the air. Therefore, carbon nano materials are oxidized by oxygen atoms excited by the discharged electron and dissolved by combustion. Carbon nanotube materials have a short life because of this structural weakness.
In addition, electron discharging elements having MIS structure and MIM structure described in JOPs 2003-145826 and 2003-140444 have a problem because the electron discharging property is not sufficient.
Because of these reasons, the present inventors recognize that a need exists for a charging device which can perform electron discharging such that corona products are not produced to prevent hazard to an image bearing member and deterioration of material discharging electrons, and a process cartridge and an image forming apparatus using the charging device.